Seals for use on container closures, such as metal crowns, rolled on caps or plastic caps are conventionally made from sealing compounds based on a PVC-plastisol system. Such compounds are usually lined into the closure as a liquid and fluxed to form the seal. Some seals have been formed using a PVC-based compound in which the compound is formed into an extruded form such as pellets which are deposited into the closure and shaped into the appropriate seal by means of a die.
Seals for container closures, such as bottle caps, must have specific characteristics in order to adequately seal a container. The seal must be soft enough so as to adapt to any possible irregularities of the container edge, such as a bottle neck, especially on a reused glass bottle. Simultaneously, the seal must be adequately elastic to maintain the seal between the closure and the container, especially when the container holds a pressurized product, such as beer or tonic. The seals must also serve as barriers to the ingress or egress of water vapor and oxygen. This is especially important in connection with the keeping qualities of beer, which is adversely affected by oxygen. Finally, the seal itself must not alter the taste of the packed product.
PVC-based compounds adequately fulfill these characteristics, though the use of PVC has been objected to because of its waste disposal problems and in particular, because of the health risks which have been claimed from the use of PVC on food packs. There has therefore been an increased demand for compounds which are free of PVC, yet have the same sealing characteristics as PVC.
Some attempts have been made to make sealing compounds based on polyethylene, though they have not gained any widespread acceptance due to their inferior characteristics as compared to PVC-based compounds. Some characteristics have been enhanced through the addition of various additives, but the additives make the compounds so expensive that they re not competitive or marketable.